Thermal overload relay



March- 9, 1943. A- Q ST|M$0N 2,313,597

THERMAL OVERLOAD RELAY Filed Nov. 8, 1941 Inventor: Allen G. Stmson,

b5 JV JMJ/7 H is ttor-neg Patented Mar. 9, 1943 THERMAL OVERLOAD RELAY Allen G. Stimson, Marblehead, Mass., assignor to General Electric Company, a corporation of New York Application November 8, 1941, Serial No. 418,372

(CL 20G-113) 1l Claims.

My invention relates to thermal overload devices and has for its object a provision of a device of this character which is reliable and eilicient in operation and simply and readily manufactured at low cost. The device is particularly well suited for the protection of multi-phase electric circuits against overloads occurring in any phase of the circuits.

'I'he simple laterally distortable thermostatic strip provided with a resistance heater leaves much to be desired in eillciency and compactness. For example, the disposition of thermosensitive material in a fiat strip is such that it is difllcult, if not impossible, to produce a compact device including such a thermosensitive element, while the use of a separate resistance heater produces an inherent and undesirable temperature diii'erentiai and time lag ln operation.

Accordingly, it is an object of my invention to provide a new and approved thermal overload device in which a relatively large amount of thermosensitive material may be disposed within a small space and arranged to provide straightline motion.

It is a further object of my invention to provide an overload device of the above character having an accurately adiustable andpoweriul snap action having similar operating characteristics in both directions of operation.

It is a still further object of my invention to provide a magnetic. snap action for a device oi the above type in which the maximum air gap is substantially independent of the displacement between the magnet and its keeper, thereby greatly enhancing the magnetic pull available from a magnet ot a predetermined size.

In accordance with the invention, I provide a thermosensltive actuating element comprising a bimetallic strip wound to form a helix of relatively small diameter, the small helix being wound to form a second helix of relatively large diameter. A switch actuating rod is disposed longitudinally of the second helix and subjected to straight-line movement along its axis by connection to one end of the iii-metallic strip. The double helical bimetallic strip is arranged to deilect in accordance with the heat developed by v an electric current passed through the strip, thereby to disengage a pair of normally engaging circuit controlling contacts. I also provide snap acting means for releasably retaining the actuating rod in either a circuit closing or a circuit opening position. Preferably the snap acting means comprises a permanent magnet cooperable with a pair of keepers attached to the actuating rod, one oi' said keepers engaging the magnet in each switching position. Themagnet is also provided with special shunting means to determine a maximum air gap which is independent of the displacement between the magnet and its keepers. Preferably, although not necessarily, two of the devices described are arranged within a single compact casing to control a plurality of phases of a multi-phase electric circuit. For example, a three-phase may be protected by providing each of two actuating rods with a pair of circuit controlling contacts arranged in two of the phases oi' a three-phase circuit, one contact of each actuating element being connected in series circuit relation with one contact of the other actuating element.

My invention itself may be better understood and its objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawing in which Fig. 1 is a perspective view, partly in section, showing a thermal overload device according to my invention and adapted for the protection of a three-phase circuit; Fig. 2 is an exploded perspective view showing details of certain portions of a preferred form of my snap acting mechanism; and Fig. 3 is a partially schematic circuit diagram showing the manner of connection oi' the device of Fig. 1 for operation in a three phase circuit.

Referring now to the drawing and particularly to Fig. 1, I have shown a thermal overload device according to `my invention mounted Within a box-like casing I0 of suitable insulating material, suchas a molded plastic compound. The casing I 0 is provided with a longitudinally-disposed dividing wall II preferably formed integrally with and of the same material as the casing. The casing i0 is also provided with suitable bosses and recesses for convenient mounting of an independent thermal overload mechanism in each of the compartments formed by the wall I I. Attached to the casing Ill at one end thereof are a plurality of iixed contacts I2, I3 and I4 and a fourth contact not shown. A pair of pivotally and resiliently mounted movable current carrying contact strips I6 and I6 are mounted upon the casing II) to connect pairs of opposite stationary contacts. The contact strip I 5 carries a movable contact at each end thereof and is biased into normal engagement with the contact I4 and the fourth contact (not shown) by a compression spring Ia. Similarly, the contact strip I6 carries a movable contact at each end thereof and is normally biased into engagement with the contacts I2 and I3 by a similar compression spring disposed between the contact strip and the casing I9.

An overload element having an actuating rod I1 connected to a transverse arm I8 of suitable insulating material is arranged to disengage the movable contacts at one end of both of the mov-' ably mounted contact strips I5 and I6, while a similar overload element having an actuating rod I9 connected to a transverse arm 20 is arranged to disengage the movable contacts at the opposite end of each of the movable contact strips I5 and I6. Since the overload elements are similar in construction, sectional views of only one element have been shown. The description of this one element will provide also a description of the other.

The actuating rod I9 is slidably mounted longitudinally of the casing Il) and passes through the casing at one end thereof to engage the transverse arm 2IJ for actuating the movable contact strips. Within the casing and wound helically about the rod I9 is a helicoidal thermosensitive bimetallic strip 25. The bimetallic strip 25 is wound into a minor helix of relatively small diameter which is then coiled to form a major helix of relatively large diameter having an axis coincident with the axis of the actuating rod I9. It will be understood that the axis of the minor helix follows a helical path about the rod I9. The double helical bimetallic strip 25 is mounted upon a pair of suitably threaded adapters or end supports 26 and 21. The end supports 26 and 21 may be made of insulating material, each being of sufficient length to extend substantially halfway through the helix. Preferably, however, if the helix is self-supporting between its ends, the supports are made of aluminum for added lightness and strength and extend inwardly for only a single turn at each end of the helix to avoid short circuiting the turns thereof. The support 26 threadedly engages the actuating rod I9 and normally remains in fixed relation thereto, while the support 21 is suitably bored to receive the actuating rod I9 loosely therein. To prevent short circuiting of the bimetallic strip 25 by the rod I9, if the adapters 26 and 21 are metallic, suitable insulation is provided between the rod I9 and the adapter 21. take the form of an annular clearance space between the rod I9 and adapter 21. If'desired. an insulating bushing 21a may be inserted in the clearance space. The support 21 is provided with a disk-like base portion by means of which it is iixedly mounted upon the insulating casing Il). As shown, the disk-like base portion of the support 21 is simply and easily assembled within the casing ID between a pair of transverse grooves 28 formed in the molded casing. Preferably the turns of the major helix are separated by spacing disks 29a of insulating material. The disks 29a are inserted part way through the helix and serve to space the turns and prevent short circuiting when the helix is collapsed. If desired, of course, a helical strip of insulating material may be in- Such insulation may terwound with the turns of the bimetallic helix.

As may be seen from the drawing, the actuating rod I9 is in the form of a. threaded bolt having a head portion engaging the transverse arm 20. Since the transverse arm 20 is at all times biased into engagement with the head of the actuating bolt I9 by the springs |50., it is unnecessary to provide more than a slidable connection between the rod I9 and the arm 20. However, it is necessary to preclude rotational movement of are held in clamping engagement.

the arm 20 with respect to the bolt I9. and for this purpose a pair of pins 29 extend from op` posite sides of the arm 20 and are slidably mounted in suitable holes formed in the end of the casing III. Similar pins are mounted on the transverse arm I8. It will be noted that the pins 29 extend only partially through the casing I0 so that they will not short circuit the strips I5 and I6 through the metallic support 21.

Within the opposite end of the casing I0 is mounted a double magnetic snap acting mechanism operable in conjunction with the actuating rod I9. This mechanism comprises a per` manent magnet mounted in fixed relation to the casing III and cooperable with a pair of keepers 30 and 3| mounted upon the actuating rod I9 on opposite sides of the magnet. Referring now particularly to Fig. 2. the permanent magnet` is preferably formed of a pair of similar substantally cubical sections 35 and 3E, although it wil be understood that if desired, the sections ma be formed integrally. In order to obtain suiii-l cient coercive force while limiting the physical proportions of the magnet. I prefer to use for the magnet an alloy comprising six to fteen per cent aluminum, twelve to thirty per cent nickel, about ve per cent copper if desired, and the remainder, of iron, such as described and claimed in Patents 1,974,274 and 1,968,569, issued February 13 and` July 31, 1934, respectively, to William E. Ruderi For convenience in mounting the magnets 35 and;` 36 centrally with respect to the actuating rod IS each magnet section is provided with a groove 31A of semicircular cross section, and the magnets. are mounted so that the grooves 31 are oppositely: disposed to form a cylindrical bore within whichs the rod I9 is slidably supported. The outer ends of the magnet sections 35 and 36 are slottedlgf-W transversely of the grooves 31 at 38 and 39, re, spectively, to provide for mounting the magnets between a pair of bolts 40 and 4I which passi transversely through the casing I0.

The magnet sections 35 and 36 are magnetizect longitudinally of the slots 38 and 39 so that nortrr and south poles are formed at opposite sides of the rectangular magnet, as indicated at Fig. 2f In order to retain the magnetic iiux within predetermined limits, regardless of the displacement between the magnets and their keepers a pair of U-shaped shunts 42 and 43 are disposed at opposite pole faces of the magnets. The shunts 42 and 43 are formed of a magnetizable material such as soft iron and are provided with side portions extending around the ends of the-magnet and defining between them predetermined small air gap 44' (Fig. 1). To hold the shunts 42 and 43 in fixed position relative to the magnet sections 35 and 3B the bases of the shunt are formed to provide at their ends upwardly-extending edges 45, 46 (Fig. 2) between which the magnet sections As may be clearly observed at Fig. 1, the shunts 42 and 43 extend beyond the sides of the magnets 35 and 36 so that the keepers 3D and 3i come into engagement only with the ends of the shunts. The keepers 39 and 3| are adjustably mounted upon the threaded portion of the actuating rod I9 by means of threaded nuts 41 and 48, respectively. The nuts 41 and 48 are machined to provide cylindrical portions extending within the bore formed by the oppositely-disposed grooves 31 in the magnets, thereby providing a slidable bearing support for one encl of the actuating rod I9.

In operation the device of Fig. 1 may be connected in a three phase circuit 50, 5|, 52 in the manner shown at Fig. 3. As shown, the conducting strip I and the contacts at opposite ends Ythereof are connected in series with one of the bimetallic helices 25 in the phase 60, while the conducting strip IS and the contacts I2 and Il are connected in series with the other helix 25 in the phase 5 I. Under normal current conditions, the current carrying strips l5 and IB are biased into engagement with their respective stationary contacts'by means oi' the springs |50. disposed between the current carrying strips and the end of casing I0. Thus, the device is in the position of Fig. l, in which the double helical thermosensitive elements are contracted and the keeper III is in engagement with the outer end of the snap action magnet shunts. Upon the occurrence of an excessive current in any phase of the circuit, for example in the phase 5|, the associated thermosensitive bimetallic strip 25 will become heated and its free end will tend to move with a straightline motion in a direction to disengage the keeper 30 from the magnets 35 and 3E. As soon as the energized thermosensitive element 25 builds up suflicient force to disengage the keeper 30, the actuating rod I9 moves longitudinally with a snap action. Preferably the parts are so constructed and arranged that the keeper 3| will come into engagement with the magnet shunt without further heating of the thermosensitive element. By this movement the connected cross arm 2u is drawn toward the end of the casing Ill thereby to depress one end of the current carrying strips I5 and |6 and disengage the circuit controlling contacts. While the actuating rod I9 is thus moving from its circuit closing to its cir cuit opening position, both keepers 30 and Il are momentarily disengaged from the magnet shunt. During this interval the magnetic ilux is shifted from its normal path through the keeper 30 to a shunt path including the sides oi' the shunts 42 and 43 and the air gap 4I. Thus it will be observed that the distance between the shunts I2 and 43 deiines a maximum air gap 44 which is constant regardless ci the displacement between the keepers and the magnets. As soon as the keeper 3| engages the' shunt, the magnetic iiux path again shifts so that the major part of the flux passes through the keeper 3|.

In view of the above detailed description it will be evident that I have provided a thermal overload device in which a relatively large amount of thermosensitive material may be simply and economically arranged within a relatively small space so that a device of large capacity may be made in a small size. Furthermore, my invention includes snap acting means having the same operating characteristic in both directions of m0- tion, that is, the double keeper construction cooperating with a single magnet is so arranged that movement of the actuating rod from either its circuit closing or its circuit opening position requires the building up of a predetermined and adjustable initial force in the thermosensitive element 25. To provide a large magnetic pull a magnet of high inherent coercive force is provided with special shunting means for deiining a maximum air gap and thereby maintaining the magnetic flux within predetermined limits. In addition to the foregoing advantages my overload device is arranged to provide direct heating of the thermosensitive element so that the device is not subject to an inherent temperature dlilerential and time delay in cooling.

While I have shown and described but one preferred embodiment or my invention by way of rence of a predetermined excessive current in said circuit, snap acting means including a permanent magnet and a keeper for retaining said movable contact in either a circuit closing or a circuit opening position, and means for maintaining the flux of said permanent magnet within predetermined limits regardless oi the maximum displacement between said magnet and keeper.

2. A thermal overload device for an electric circuit comprising fixed and relatively movable contacts in normal engagement, thermosensitive actuating means arranged to disengage said contacts with a straight line motion upon occurrence of a predetermined excessive current in said circuit, snap acting means including a permanent magnet and a keeper for retaining said movable contact in either a circuit closing or a circuit opening position, and means independent ofsaid keeper for providing a return path of a predetermined maximum reluctance for the flux of said permanent magnet.

3. A thermal overload device for an electric circuit comprising fixed and movable contacts in normal engagement, an actuating member for said movable contact, thermosensitive means connected to move said actuating member with a straight line motion thereby to disengage said contacts upc.. the occurrence of a predetermined excessive current in said circuit and to reengage said contacts upon the occurrence of the predetermined lower current of said circuit, snap acting means for retaining said actuating member in either a circuit closing or a circuit opening position, said snap acting means including a fixed permanent magnet and a movable keeper connected to said actuating member, and magnetic shunting means for said permanent magnet arranged to determine a maximum air gap independent of the maximum displacement of said movable keeper with respect to said magnet.

4. A thermal overload device for an electric circuit comprising ixed and movable contacts in normal engagement, an actuating member for said movable contacts. thermosensltive means connected to move said actuating member with a straight line motion thereby to disengage said contacts upon the occurrence of a predetermined excessive current in said circuit and to reengage said contacts upon the occurrence of a predetermined lower current in said circuit. snap acting means for retaining said actuating member in either a circuit closing or a circuit opening position, said snap acting means including a fixed permanent magnet of substantially rectangular configuration and a pair of movable keepers of magnetic material alternatively engageable with opposite faces of said magnet, and iixed shunting means of magnetizable material positioned to determine a iixed air gap through which the flux of said magnet is shunted when both said keepers are disengaged from said magnet.

5. A thermal overload device for an electric circuit comprising xed and movable contacts in normal engagement. an actuating member lor said movable contacts, thermosensitive means connected to move said actuating member with a straight line motion thereby to dlsengage said contacts upon the occurrence of a predetermined excessive current in said circuit and to reengage said contacts upon the occurrence oi a predetermined lower ,current in said circuit, snap acting means for retaining said actuating member in either a circuit closing or a circuit opening position, said snap acting means including a fixed permanent magnet of substantially rectangular conguration and a pair of movable keepers of magnetic material connected to said actuating member in spaced relation. said actuating member and said magnet being so positioned that said keepers are arranged alternatively to engage oppos-ite sides of said magnet, said magnet being magnetized in a direction substantially parallel to the plane of said first-named sides so that its intermediate opposite sides are oi opposite polarities, and a pair of U-shaped shunts of magnetic material having side portions extending around the ends ot said magnet and defining a ilxed air gap therebetween thereby to provide a shunt path of predetermined magnetic reluctance for the flux of said magnet regardless of the relative positions of said magnet and keepers` 6. A thermal overload device for arl electric circuit comprising xed and movable contacts in normal engagement, an actuating rod connected to move said movable contacts, a helicoidal bimetallic strip coiled about said actuating rod to form a helix, one end of said bimetaliic strip being ilxedly mounted and the other end being attached to said actuating rod for moving said rod with a straight line motion to disengage said contacts upon the occurrence of a predetermined excessive current in said circuit and to reengage said contacts upon the occurrence of a predetermined lower current in said circuit, snap acting means for retaining said actuating 'rod in either a circuit closing or a circuit opening position, said snap acting means including a tlxed permanent magnet and a pair o! movable keepers of magnetic material connected to said actuating rod in spaced relation, said keepers being arranged alternatively to engage opposite'sides of said magnet, and magnetic shunting for said permanent magnet arranged to determine a maximum air gap independent of the maximum displacement of said movable keepers with respect to said magnet.

7. A thermal overload device lor an electric circuit comprising a base of insulating material, relatively ilxed and movable contacts mounted upon said base and biased into normal engage ment, an actuating rod slidably mounted upon said base and arranged to engage said movable contact and open said circuit upon the occurrence of a predetermined excessive current in said circuit, a double helical bimetallic strip coiled about said actiatlng rod and having one end connected to sai base and the other end connected to said rod thereby to move said rod relative to said base with a straight line motion, a permanent magnet of substantially rectangular conilguration having a transverse bore therein, said magnet being so positioned that said actuating rod passes freely through said bore and having opposite sides magnetlzed transversely of said rod, a pair of U-shaped shunts of magnetic material arranged transversely with respect to said rod and extending over the ends o! said magnet between said opposite sides to denne therebetween a predetermined maximum air gap and to provide a shunt path of maximum reluctance for the iiux ot said magnet, and a pair of movable keepers of masnetizable material mounted upon said actuating rod in spaced relation and arranged alternatively to engage and magnetically to shunt said U-shaped shunts.

8. A double snap acting device comprising a movable actuating member, a permanent magnet soY and relatively movable magnetizable means operable in conjunction with said movable member selectably and releasably to retain said member in either of two spaced positions, and means 1n dependent oi' said magnetizable means providing a return path o! predetermined maximum reluctance for the flux of said permanent magnet thereby to retain said flux within predetermined limits regardless of the displacement between sald permanent magnet and said magnetizable means.

9. A double snap acting device comprising a movable member, a permanent magnet and a pair of magnetizable keepers, said magnet and keepers being relatively movable in conjunction with said movable member, and said keepers being arranged for alternative engagement with opposite faces of said magnet selectably and releasably to retain said movable member in either of two spaced positions, and iixedshunting means of magnetizable material positioned to determine a ilxed air gap Vthrough which the ilux oi said magnet is shunted when both said keepers are disengaged from said magnet.

10. A double snap acting device for a movable actuating member comprising a permanent magnet. a pair of magnetizable keepers connected to said actuating member in spaced relation, said keepers being arranged on opposite sides of said magnet alternatively to engage opposite faces thereof selectabiy and releasably to retain said actuating member, in either of two spaced positions, and magnetic shunting means for said permanent magnet arranged to determine a. maximum air gap independent of the displacement ci saitd movable keepers with respect to said magne l1. A double snap acting device for a movable actuating member comprising a permanent magnet ci substantially cubical configuration, a pair of movable keepers of magnetizable material connected to said actuating member in spaced relation on opposite sides of said magnet, said actuating member and said magnet beng so positioned that said keepers are arranged alternatively to engage opposite faces of said magnet, said magnet being magnetized in a direction substantially parallel to the plane of said races so that its intermediate opposite faces are of opposite polarities, and a pair of U-shaped shunts oi magnetic material having side portions extending around the ends of said magnet and delining a iixed air gap therebetween thereby to provide a shunt path of predetermined magnetic reluctance tor the flux or said magnet regardless of the relative positions of said magnet and keepers.

ALLEN G. STIMSON.

CERTIFICATE op CORRECTION. Patent No. 2,5151597' March 9, 1515.

ALLEN o. sTInsoN.

It is herebjr certified thnt error4 appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, 11n 29, for the patent number "1,97h,2?l; read --1,9I+7,2714,; page I+, second column, line 5T, claim 11, for *bang* reed --being; and

that the said Letters Patent should be reed with this correction thereinY that the same may conform tothe record o! the cese in the Patent Office.

signed and sealed this 20th any of Apr-11, A. n. i9LL5.

. Henry van Axsdmle,l (Seal) Acting Commissioner of Patents. 

